1. Field of the Invention
The present invention relates to an optical scanning apparatus for scanning a scanning object by irradiating light, such as plural laser lights whose wavelengths are different from each other, and also relates to an information reading apparatus for reading information by scanning a reading object, and also relates to an information recording apparatus for recording information read by scanning a recording object.
2. Description of the Related Art
An optical scanning apparatus is used in the so called color copy or the like. For instance, in the color copy process, the optical scanning apparatus irradiates plural laser lights whose wavelengths are different from each other, such as a red laser light, a blue laser light, a green laser light, etc., onto a scanning object, and reads pictures etc recorded on the scanning object. Further, the pictures etc read by the optical scanning apparatus is transferred to a transference object in the color copy process.
In the scanning operation of the optical scanning apparatus, laser lights are emitted from laser devices. Each emitted laser light is passed through a collimator, and thus, each laser light becomes a parallel luminous flux. Thereafter, all of the optical paths of the laser lights are substantially equalized to each other by a dichroic mirror etc. Further, each laser light is irradiated to a rotating polygon mirror, and therefore, each laser light is irradiated onto the scanning object though an image formation lens. The image formation lens is constructed by the so called f.theta. lens. The f.theta. lens has the characteristic that image height (image size) h is in proportion to the product of angel .theta. of incidence of a laser light and focal distance f (i.e., h=f.times..theta.). By passing through the image formation lens, each laser light is converged on the scanning object respectively. Thus, the scanning object is scanned by each converged laser light.
In the aforementioned optical scanning apparatus, all of the optical paths of the laser lights are substantially equalized to each other, and all of the laser lights are irradiated to the single image formation lens, and then, are converged on the scanning object. Therefore, a chromatic aberration is generated on the scanning object because of differences between wavelengths of the laser lights.
Here, the chromatic aberration indicates aberration of the image formation lens, and it indicates that a position of an image formed by one laser light is different from that formed by others. The chromatic aberration includes a longitudinal chromatic aberration and a chromatic aberration of magnification. The longitudinal chromatic aberration indicates that a position of an image formed by one laser light is different in the optical axis from that formed by others. Namely, the longitudinal chromatic aberration indicates that a focal position of one laser light is different from that of others. On the other hand, the chromatic aberration of magnification indicates that a size of an image formed by one laser light is different from that formed by others. In addition, the cause of chromatic aberration is that a reflective index of the laser lights are different from each other in accordance with the wavelengths of the laser lights, when the laser lights whose wavelength are different from each other are passed through the single image formation lens.
On the other hand, in order to scan the scanning object correctly, it is required to compensate the chromatic aberration. Here, in order to compensate the chromatic aberration of magnification, an improvement of a construction of an image formation lens (f.theta. lens) was proposed. This is described in a Japanese patent application laid open, No. hei 7-191261. However, the longitudinal chromatic aberration is disregarded because the required resolution is not high in the conventional optical scanning apparatus. Therefore, in the resolution of the conventional optical scanning apparatus, if sizes of light spots are different from each other by the longitudinal chromatic aberration, the differences between the sizes of the light spots are in the limits of a focal depth of an image formation lens. That is to say, in the conventional optical scanning apparatus, since the focal depth is deep, it is not required to compensate the longitudinal chromatic aberration.
However, recently, the demand for the high resolution optical scanning apparatus is increasing. For example, in order to realize the resolution of 600 dpi (dot per inch), it is required that a diameter of a light spot of each laser light is reduced to 60 [.mu.m]. Further, in order to reduce the diameter of the light spot to this value, it is required that the number of an aperture of the image formation lens is increased. If the number of the aperture of the image formation lens is increased, differences between focuses of laser lights in a curvature of an image surface (i.e., the longitudinal chromatic aberration) are larger than the size of the light spot. As a result, if the longitudinal chromatic aberration is disregarded, the diameter of the light spot of each laser light cannot be reduced, and the size of the light spot is large. Thus, the high resolution scanning cannot be realized.
Further, in order to realize the high resolution scanning, it is required to more sufficiently compensate the chromatic aberration of magnification in response to a reduction of the size of the light spot.